We are exploring the biochemical and biological properties of defective interfering particles (DI particles) of enveloped RNA viruses. We are examining the effects of DI particles on homologous virus infections both in vitro and in vivo with particular emphasis upon possible attentuating effects of DI particles on acute infections and upon the possibiblity that DI particles might play a major role in facilitating long term persistent infection by otherwise virulent viruses both in vitro in carrier cultures and in vivo in animals. The viruses being studied are VSV, rabies, measles, mumps and influenza. We are also studying the biochemistry of B virions and T particles and the biochemical alterations these cause during cellular infection.